1. Field of the Invention
This invention relates to computer systems, and more particularly, to methods and apparatus for detecting one of two different input voltage levels provided as input to an integrated circuit.
2. History of the Prior Art
A major trend in the manufacture of personal computers is to provide portable computers which function for extended periods. This has led to attempts to reduce the power used by portable computers. In order to reduce power consumption and extend battery life, much of the integrated circuitry used in personal computers is being redesigned to run at lower voltage levels. This reduces the power usage and allows more components to be placed closer to one another in the circuitry. The circuitry and components used in portable computers are being designed to operate at voltages levels such as five volts. Certain components of portable computers are even being designed to run at even lower voltages such as three volts. These lower voltage levels help a great deal to reduce the power needs of such computers.
During this period of development of lower powered computers, many system designers are designing systems using all five volts components while others are designing systems using some components which operate with three volts and others which operate with five volts. Thus, a particular portable computer may have available both three and five volts sources. Because of the many different combinations of components available in the present market, it is desirable to provide circuitry which is capable of functioning in systems providing either or both five or three volt external sources.
One component which is finding wide use in portable computers the flash electrically-erasable programmable read-only memory (EEPROM) array. Flash EEPROM memory arrays may be used to provide a new form of long term random access storage which may replace electro-mechanical hard disk drives. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is described in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. Such an array provides a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is important. However, these flash EEPROM memory arrays also require much higher voltages for their operations than can be provided directly by the batteries of low powered portable computers.
In order to provide the higher voltages necessary to program and erase flash EEPROM memory arrays, charge pump circuitry is being utilized. Charge pumps typically increase voltage available by pumping the voltage to a higher level. In the typical case, a five volt external source is pumped to twelve volts to provide voltages for programming and erasing. Usually the five volts is sufficient for reading the array. However, designers of computers which have available a three volt source which may be pumped to five volts and used for reading the flash EEPROM memory array. Thus, in such circuits, it is necessary to know the level of voltage available in order to know whether to pump the external voltage to a higher value which may be used for reading the array. Many other purposes exist for the use of circuitry which provides an indication of a value of an external voltage so that circuitry may be optimized for use with the particular voltage. Moreover, it is especially desirable if the circuitry providing such an indication itself functions at very low power levels in order to enhance the power savings characteristics of a portable computer.